1. Field of the Invention
The present disclosure relates to a method for producing (manufacturing) a semiconductor device.
2. Description of the Related Art
Among known light-emitting diodes (LEDs) which are semiconductor light-emitting devices that use GaN compound semiconductors and the like, there are those which have homojunction structures including p-type compound semiconductors directly joined to n-type compound semiconductors, those which have double heterojunction structures including active layers sandwiched between p-type and n-type compound semiconductors, and those which have quantum well junction structures including quantum well layers sandwiched between p-type and n-type compound semiconductors. Among these junction structures, double heterojunction structures and quantum well junction structures are advantageous for realizing LEDs exhibiting higher luminance, depending on the type of the matrix compound, the type of dopants added to the matrix, the stoichiometry of the compound, etc.
LEDs having such junction structures have been widely produced by, for example, forming junction structures of compound semiconductors constituting the LEDs on buffer layers disposed on light-transmitting supporting substrates such as sapphire substrates. However, when LEDs are formed by using such supporting substrates, the difference in refractive index between the sapphire supporting substrates and compound semiconductor layers (e.g., GaN layers) formed on the sapphire supporting substrates by crystal growth is large. Thus, light is reflected (confined) at the interface between the substrates and the compound semiconductor layers, resulting in lowering of the output efficiency of LEDs' emission (LEDs' emission efficiency). Thus, LEDs having sufficiently high luminance have been difficult to obtain.
In order to enhance the emission efficiency of LEDs, a technique called “laser lift-off” has been proposed and developed which separates the supporting substrates composed of sapphire or the like from the junction structures of the LEDs (refer to Japanese Unexamined Patent Application Publication No. 2007-19511). According to the laser lift-off technique, in general, a junction structure for LEDs is formed on a sapphire supporting substrate, and another supporting substrate is bonded to a surface of the junction structure opposite to the sapphire supporting substrate. Then a laser beam is irradiated to the junction structure through the sapphire supporting substrate to heat the compound semiconductor layer bonded to the sapphire supporting substrate and to thereby separate the junction structure of the LED from the sapphire supporting substrate at the interface between the sapphire supporting substrate and the compound semiconductor layer.
Another technique for enhancing the emission efficiency of LEDs involves forming a free (self)-standing substrate structure using instead of the sapphire substrate a supporting substrate composed of the same compound as the compound semiconductor used in the LED (e.g., in the case where an LED uses GaN, a GaN substrate is used). According to this technique, the decrease in emission efficiency caused by the difference in refractive index between the supporting substrate and the compound semiconductor layer can be eliminated.
However, according to the conventional laser lift-off technique, since separation takes place at the interface between the supporting substrate and the compound semiconductor all parts of which are bonded onto the whole surface of the supporting substrate, an intense laser beam must be used to impart high energy to the compound semiconductor layer. As a result, the heat input (thermal budget) increases significantly. Then thermal stresses readily cause breaking such as cracks, deformations, structural defects, etc., in the compound semiconductor layer separated from the supporting substrate or the upper layers on the compound semiconductor layer. As a result, the product yield tends to decrease.
Meanwhile, supporting substrates composed of the same material as the compound semiconductors constituting LEDs are generally very expensive. For example, if GaN substrates are used, their costs are at least twenty times larger than the cost of sapphire supporting substrates. This is not economically favored for mass production.
Therefore, a method for producing a semiconductor device that can achieve improved production yield by preventing damage on compound semiconductors and junction structures during production processes and offer low-cost, high-luminance light-emitting elements by preventing the increase in production cost is desired.